The once taboo topic of cannibalism is actually perfectly natural in the animal kingdom, changing what we know about evolution.

I was knee-deep in a temporary pond that seemed to be composed of equal parts rainwater and cow dung when the cannibals began nibbling on my leg hair.

“If you stand still for long enough, they’ll definitely nip you,” came a voice from the shore.

“They” were cannibalistic spadefoot toad larvae, commonly known as tadpoles. The warning had come from David Pfennig, a biology professor at the University of North Carolina who had been studying these toads in Arizona’s Chiricahua Mountains for more than 20 years.

At Pfennig’s invitation, I had arrived at the American Museum of Natural History’s Southwestern Research Station in mid-July — just after the early summer monsoons had turned cattle wallows into nursery ponds and newly hatched tadpoles into cannibals. But the real reason I had come to the ancestral land of the Chiricahua Apaches wasn’t because the tadpoles were eating each other. It was because some of them weren’t eating each other. In fact, when this particular brood had hatched about a week earlier, they were all omnivores, feeding on plankton and the suspended organic matter referred to in higher-class journals as “detritus.”

Then, two or three days later, something peculiar took place. Some of the tiny amphibians experienced dramatic growth spurts, their bodies ballooning in size overnight. Now, as I waded, scoop-net in hand, through Sky Ranch Pond (a slimy-bottomed mud hole with delusions of grandeur), the pumped-up proto-toads were four or five times larger than their poop-nibbling brethren.

“These look like two different species,” I said, examining a handful of tadpoles that I’d just scooped up. I also noted that the larger individuals were light tan in color while the little guys had bodies flecked with dark green.

“Initially, people thought they were different species,” Pfennig replied.

Patricia J. Wynne

Using a magnifying glass to get a better look at my squirmy captives, I saw the differences went beyond body size and color. The larger tadpoles were also sporting powerful tails and serious-looking beaks.

“Yikes, nice choppers,” I commented, always the scientist.

“They’re made of keratin,” Pfennig said. That’s the same tough, structural protein found in our nails and hair.

Later, while comparing the two tadpole morphs — larvae transforming into toads — under a dissecting microscope, I saw that behind a set of frilly lips, the flat keratinous plates (which had worked fine for detritus dining) had been transformed into a jack-o-lantern row of sharp-edged teeth in the cannibalistic forms. The jaw muscles were also significantly enlarged in the cannibals, especially the jaw-closing levator mandibulae, whose bulging appearance reminded me of a kid with six pieces of Dubble Bubble jammed into each cheek. Studies had shown that myofibers, the cells making up these muscles, were larger and greater in number — producing a more powerful bite. Of course, the extra bite force was necessary because, beyond latching onto the occasional unshaved human leg, these critters were using bulked-up bodies and the weaponry that accompanied them to consume their omnivorous pondmates.

Patricia J. Wynne

Over a three-day period, I watched and captured tadpoles in bodies of water that ranged from tire-carved puddles to bovine swimmin’ holes of the double-wide Olympic variety. From the researchers, I learned a great deal about the three species of the amphibious spadefoot toads that laid their eggs in such dangerously unpredictable conditions. Much of this information centered on the ecology, behavior and evolution of these creatures. Of course, the cannibalism angle was there, too, although these researchers treated that behavior as perfectly normal.

Until relatively recently, though, and with very few exceptions, cannibalism in nature would have been regarded as anything but normal. As a result, until the last two decades of the 20th century, few scientists spent time studying a topic thought to have little, if any, biological significance. Basically, the party line was that cannibalism, when it did occur, was either the result of starvation or the stresses related to captive conditions.

It was as simple as that.

Or so we thought.

Perfectly Natural

In the 1970s, Laurel Fox, a University of California, Santa Cruz, ecologist, took some of the first steps toward a scientific approach to cannibalism. She had been studying the feeding behavior of predatory freshwater insects called backswimmers. Fox determined that while the voracious hunters relied primarily on aquatic prey, “cannibalism was also a consistent part of their diets.” Soon after, she began compiling a list of scientific papers in which cannibalism had been reported. Although there turned out to be hundreds of references documenting the behavior in various species, no one had linked these instances together or come up with any generalizations regarding the behavior. By the time Fox’s review paper came out in 1975, she had concluded cannibalism was not abnormal behavior at all, but a completely normal response to a variety of environmental factors.

She also determined that cannibalism took place in every major animal group, including many long considered to be herbivores — like butterflies. She emphasized that cannibalism in nature also demonstrated a complexity that seemed to match its frequency. Fox suggested that the occurrence of cannibalism in a particular species wasn’t simply a “does occur” or “doesn’t occur” proposition, but was often dependent on variables like population density and changes in local environmental conditions. She even followed cannibalism’s environmental connection onto the human branch of the evolutionary tree.

Patricia J. Wynne

After pondering reports that humans practicing non-ritual cannibalism lived in “nutritionally marginal areas,” she proposed that consuming other humans might have provided low-density populations with 5 to 10 percent of their protein requirements. Conversely, she suggested cannibalism was rare in settlements where populations were dense enough to allow for the production of an adequate and predictable food supply.

In 1980, ecologist and scorpion expert Gary Polis picked up the animal cannibalism banner and began looking at invertebrates that consumed their own kind. Like Fox, he noted that while starvation could lead to increases in the behavior, it was certainly not a requirement. Perhaps Polis’ most important contribution was assembling a list of cannibalism-related generalizations under which most examples of invertebrate cannibalism could be placed: 1) Immature animals get eaten more often than adults; 2) Many animals, particularly invertebrates, do not recognize individuals of their own kind, especially eggs and immature stages, which are simply regarded as a food source; 3) Females are more often cannibalistic than males; 4) Cannibalism increases with hunger and a concurrent decrease in alternative forms of nutrition; and 5) Cannibalism is often directly related to the degree of overcrowding in a given population.

Polis emphasized that these generalizations were sometimes found in combination, such as overcrowding and a lack of alternative forms of nutrition (a common cannibal-related cause and effect), both of which now fall under the broader umbrella of “stressful environmental conditions.”

In 1992, evolutionary biologists Mark Elgar and Bernard Crespi edited a scholarly book on the ecology and evolution of cannibalism across diverse animal taxa. In it, they refined the scientific definition of cannibalism in nature as “the killing and consumption of either all or part of an individual that is of the same species.” Initially the researchers excluded instances where the individuals being consumed were already dead or survived the encounter — the former they considered to be a type of scavenging. Eventually, though, they decided these were variants of cannibalistic behavior observed across the entire animal kingdom.

As the study of cannibalism gained scientific validity in the 1980s, more and more researchers began looking at the phenomenon, bringing with them expertise in a variety of fields. From ecologists, we learned cannibalism was often an important part of predation and foraging, while social scientists studied its relationship to courtship, mating and even parental care. Anatomists found strange, cannibalism-related structures to examine (like the keratinous beak of the spadefoot toad) and field biologists studied cannibalism under natural conditions, thus countering the previous mantra that the behavior was dependent on captivity.

The air temperature had risen to 95 degrees Fahrenheit, which kept most of the area’s terrestrial denizens hiding in shade or below ground. But the inhabitants of Horseshoe Pond reminded me of sugared-up kindergartners tearing around a playground (albeit with fewer legs and more cannibalism). By this time, I had already begun to see distinct patterns of behavior in the spadefoot tadpoles that motored hyperactively just below the water’s surface.

Until relatively recently, though, and with very few exceptions, cannibalism in nature would have been regarded as anything but normal.

I noticed that the smaller, omnivorous morphs generally stuck to the shallows bordering the shoreline. They buzzed through the brown water in a non-stop, seemingly random quest for food, changing direction abruptly and often. One explanation for the patternless swimming behavior became apparent as I waded farther away from the shore, for here in the deeper water was the realm of the cannibals. I stood quietly and watched as hundreds of conspicuously larger tadpoles crisscrossed the pond, making frequent excursions from the deeper water toward the shore in a relentless search for prey.

So why did certain spadefoot larvae exhibit cannibalistic behavior? There certainly seemed to be enough organic matter suspended in these algae-tinted ponds to feed the entire brood and more.

As I spoke to Pfennig and his team of researchers, I learned that the answer was directly linked to the aquatic environments in which the adult amphibians laid their eggs. Formed by spring and early-summer monsoons, the transient ponds frequented by the spadefoots are often little more than puddles, and as such they can evaporate quite suddenly in the hot, dry environment of southeastern Arizona. Natural selection, therefore, would favor any adaptations enabling the water-dependent tadpoles to get out of the pool as quickly as possible (i.e., to grow legs). In this instance, the phenomenon that evolved can be filed under the rather broad ecological heading of phenotype plasticity: When changing environmental conditions allow multiple phenotypes (observable characteristics or traits) to arise from a single genotype (the genetic makeup of an organism).

The selection pressure lies in the temporary nature of the brood ponds, where the eggs are deposited and hatch, and where the tadpoles develop into toadlets. The period from egg to juvenile toad normally takes around 30 days — unless, that is, the pond dries out first, killing the entire brood. In response to this particular environmental selection pressure, what evolved was a means by which some of the tadpoles can mature in about two-thirds of the time. The increased growth rate occurs because the cannibal larvae are getting a diet high in animal protein as well as a side order of veggies, the latter in the form of nutrient-rich plant matter their omnivorous prey had consumed during what turned out to be their last meal.

Though the story of spadefoot toad cannibalism has been well researched, it is not fully resolved. No one has yet been able to identify the precise stimulus within these brood ponds that triggers the appearance of the cannibal morphs.

However, Pfennig and his co-workers did previously work on a completely different cannibalism-triggering stimulus in another amphibian. And this one happened to be one of North America’s most spectacular species.

The Small Get Eaten

Tiger salamanders (Ambystoma tigrinum) are the largest salamanders in the United States, reaching lengths of up to 13 inches. These thick-bodied, sturdy-limbed urodelans are widespread across much of the country. Their markings, yellow blotches against a black body, make them easy to identify, but they are rarely seen in the open except during annual marches to a nuptial pond. Tiger salamander eggs are laid in the late winter or early spring, and like other salamanders, and their cousins the frogs and toads, their larvae are fully aquatic with external gills and fishlike tails. They typically feed on zooplankton and other micro-invertebrates, but under certain environmental conditions, a small percentage develop traits that include huge heads, wide mouths and elongated teeth. Consequently, these toothy individuals exploit larger prey, among them other tiger salamander larvae.

Pfennig and his colleagues set up lab experiments on fertilized A. tigrinum eggs to investigate the stimuli that set these changes into motion. First, the researchers determined that the cannibal morphs only developed when larvae were placed into crowded conditions. Next, they used a variety of experiments to see whether the larval transformation might be triggered by visual cues (that didn’t work), smell (nope) or touch.

“It looks like they had to have the tactile cues,” Pfennig told me. “There’s something about bumping into each other that triggers the production of the cannibals.”

Patricia J. Wynne

Immature animals get eaten far more often than adults, and this makes larvicide (or infanticide) the most common form of cannibalism in the animal kingdom. Intuitively, it doesn’t seem logical to eat the next generation, but the behavior can make evolutionary sense for several reasons. Young animals not only provide a valuable source of nutrition, but in most species they’re relatively defenseless. So they present instant nutritional benefits but little or no threat to larger members of the same species, most of which are invulnerable to attacks from immature forms.

But beyond acquiring a meal, cannibalism enables individuals of some species to accelerate their developmental process, as we saw with spadefoot toads, allowing them to quickly outgrow a stage in which they might be preyed upon or perish due to unpredictable environmental conditions. In species like the flour beetle (Tribolium castaneum), the behavior may also impart a reproductive advantage, since studies have shown that cannibalistic individuals produce more eggs than non-cannibals.

Finally, many animals maintain specific territories, within which they are intolerant to the presence of conspecifics (i.e., members of the same species). According to Polis, crowding increases the frequency with which individuals violate the space of others. By reducing overcrowded conditions, cannibalism can serve to decrease the frequency of territory violations.

The Cannibalism Catch

There are also serious drawbacks to being a cannibal.

In all likelihood, the most significant of these is a heightened chance of acquiring harmful parasites or diseases from a conspecific. Both parasites and pathogens are often species-specific and many of them have evolved defenses to defeat their host’s immune defenses. As a result, predators that consume their own kind run a greater risk of picking up a disease or a parasite than predators that feed solely on other species. In the most famous example of cannibalism-related disease transmission, the Fore people of New Guinea were nearly driven to extinction as a result of their ritualized consumption of brains and other tissues cut from the bodies of their deceased kin — kin who had been infected by kuru, an incurable and highly transmissible neurological disease.

Cannibals — whether microbes or Methodists — who eat their own relatives can also experience decreases in a measure of evolutionary success known as inclusive fitness, in which the survival of an individual’s genes, whether they’re from an offspring or a collateral relative (like a brother or cousin) is the true measure of evolutionary success. A cannibal that consumes its own offspring, siblings or even more distant relatives, removes those genes from the population and reduces its own inclusive fitness. Since this is bad juju, natural selection should favor cannibals that can discriminate between kin and non-kin. In many instances, this is exactly what happens.

Pfennig and his team found that their study subjects could recognize cues associated with their kin that were absent in non-kin.

“Most examples would fall under the heading of ‘the armpit effect,’ ” Pfennig told me. “Here, an individual forms a template for what its kin smell like based on what its own smell is.” He used the example of a species of paper wasps that regularly raid the nests of conspecifics to provide food for their own broods. In these species, individuals learn that “if an individual smells like your nest or burrow. . . you don’t eat them.”

Similarly, tiger salamander larvae are more likely to eat the larvae of unrelated individuals. Pfennig explained that he and his colleagues determined this experimentally by “preventing them from being able to smell.”

“How did you do that?” I wondered, envisioning a team of micro-surgeons hovering over a tiny, amphibious patient. Irrigation please, Nurse. Can’t you see this patient is dehydrating?

“By applying superglue under their [nostrils],” he replied.

“Oh, right,” I said with an uncomfortable laugh, before Pfennig assured me the condition was temporary.

If you’re wondering whether or not spadefoot toads avoid eating their kin, Pfennig told me omnivores associate preferentially with their siblings, whereas cannibals generally school only with non-siblings. In close encounters of the bitey kind, cannibal tadpoles release siblings unharmed and consume non-relatives. In the lab, though, apparently all bets are off if the cannibals are deprived of food and then placed in a tank with other tadpoles. In these cases, starvation becomes the great equalizer, and both kin and non-kin are eaten.

Darwinian Twist

I wondered whether H.G. Wells knew about cannibal morphs when he wrote The Time Machine in 1895. In Wells’ classic novel, the Time Traveler encounters two human species: the child-sized and docile Eloi, and the brutish Morlocks, who raise the Eloi in order to feed upon them. Wells explained the Morlocks’ cannibalistic behavior by suggesting that they were once members of a worker class, toiling underground for lazy, upper-class surface-dwellers. The Time Traveler speculates that a food shortage (i.e., an environmental change) forced the subterraneans to alter their diets — at first rats, but ultimately something a bit larger. This behavior resulted in a race of hulking cannibals, feeding on the surface-dwellers, whose own evolutionary path would produce the sheeplike Eloi, pampered, well-fed and eventually slaughtered for food.

Patricia J. Wynne

Although the Eloi-Morlock relationship was clearly meant to serve as a cautionary tale of the horrors of class distinction, Wells imagined a biological phenomenon remarkably similar to what scientists like Pfennig and his colleagues are working on today.

What these scientists hypothesize goes far beyond the realm of cannibalism and into the very mechanisms of evolution itself. Their claim is that the appearance of new traits in a population, generally regarded as a first step toward the evolution of new species, can occur by means other than the accumulation of micromutations (i.e., small-scale or highly localized mutations), the classic mechanism by which new traits, and eventually new species, are thought to appear. Some researchers now believe that given generations, novel traits originating as examples of phenotypic plasticity have the potential to produce separate species.

Innocent but Gory

In the end, cannibalism makes perfect evolutionary sense. If a population of spiders has lots of males from which a female can choose, then cannibalizing a few of them could increase Charlotte’s overall fitness by increasing the odds that she can raise a new batch of spiderlings. On the other hand (and in spiders there are eight of these to choose from), in a population where males aren’t plentiful or where the sexes cross paths infrequently, cannibalizing males would likely have a negative impact on a female’s overall fitness by decreasing her mating opportunities.

As a zoologist, I find this kind of dichotomy pleasing, since it’s logical and appears to be more or less predictable in occurrence. In nature, as far as cannibalism is concerned, I’ve found no gray areas, no guilt and no deception.

There is only a fascinating variety of innocent — though often gory — responses to an almost equally variable set of environmental conditions: too many kids, not enough space, too many males, not enough food.